Combustion devices based on hydrocarbon fuels are widely used to provide thermal, mechanical or electric energies. For example, fireplaces, ovens, furnaces, and boilers have been installed and used in commercial and residential buildings to provide heat, hot water, and other conveniences. Ideally, complete combustion occurs when hydrocarbon compounds in the fuel exothermically react with oxygen in the air to produce water vapor and carbon dioxide. Furnace systems are designed to run the combustion reaction with an excess of oxygen so that complete combustion can take place and maximum amount of heat may be released from hydrocarbon fuels.
On the other hand, a combustion reaction in which carbon monoxide (CO), a poisonous gas, is formed from a hydrocarbon is an incomplete combustion or a partial combustion. Incomplete combustion occurs when there is an insufficient amount of oxygen to react with the hydrocarbon. In addition, incomplete combustion can adversely affect the function of a combustion device, such as by decreasing its efficiency and heat output. CO is also formed from quenching a combustion process. Because of its possible adverse effects on the combustion device, it is desirable to monitor CO emissions continuously.
Traditionally, residential furnaces do not have a detection system for directly monitoring the concentration of carbon monoxide or other gasses present in combustion products due to feasibility including high cost and limited durability relative to a residential furnace. Pressure switches provide a mechanism to ensure proper airflow in furnaces. The pressure switches are only activated when a proper amount of airflow is reached. In the event of insufficient airflow the furnace shuts down. As a result, the pressure switches only deactivate the furnace system if there is an air blockage or starvation of combustion air.
Most commercially available gas sensors are generally not used in a flue gas stream of a combustion device. For example, hydrogen and helium sensors have been used in chiller tanks under negative pressure to detect any leakage existing on the exterior shell of the tanks. Similarly, household or industrial CO detectors based on liquid electrolyte are also known but generally operate at room temperature and are not suitable for use in most high temperature flue gas environment. Recently, smaller and less expensive CO sensors suitable for use in a flue gas stream, such as those disclosed in the co-pending U.S. Patent Application Publication No. 2010/0009304, have been developed and used in residential furnaces. Similar oxygen, carbon dioxide and hydrocarbon gas sensors are also functional for the purposes of detecting incomplete combustion.
One problem associated with existing flue gas sensors is premature sensor failure. For example, while the average design life of a residential furnace is about twenty years, existing flue gas sensors generally have a substantially shorter life span, with some sensor failure occurring within two to three years of operation. The disparity between the life of the sensor and that of the furnace not only requires frequent service or replacement of the sensor, but also significantly affect the safe operation of the furnace, especially toward the end of the furnace life when incomplete combustion and excessive CO emission are most likely to OMIT.
Hence, there is a need for a flue gas sensor with improved reliability and durability over existing sensors. Moreover, there is a need for a flue gas sensor having longer life spans than existing sensors without sacrificing its gas sensing performance.